A general engine that is used as a driving force source for a vehicle has such a characteristic that the output torque increases with an increase in rotation speed. In contrast, driving force that is required of a vehicle is usually relatively large in the case of a low vehicle speed, and is relatively small in the case of a high vehicle speed. That is, when the vehicle that uses the engine as a driving force source travels, torque having an opposite tendency to the output characteristic of the engine is required. When the engine is operated, an operating point at which the engine has a high efficiency is limited. Therefore, in the vehicle that uses the engine as a driving force source, a transmission is mounted. The transmission is able to change the speed ratio as needed. By setting the speed ratio as needed with the transmission on the basis of a traveling state of the vehicle, such as a vehicle speed and an accelerator operation amount, a required driving force is obtained, and the engine is operated at an operating point having a high efficiency.
Among transmissions that are mounted on the thus configured vehicle, a stepped transmission that sets the speed ratio stepwisely for each speed position has a step between the set speed ratios, so the engine cannot be constantly operated at an operating point having a high efficiency. For example, when the rotation speed of the engine at an operating point having a high efficiency is a rotation speed that can be set at a speed ratio between two speed positions, the efficiency decreases in an operating state in a period from one of the speed positions to the other one of the speed positions. Therefore, in recent years, a vehicle on which a continuously variable transmission is mounted instead of the stepped transmission has become widespread. The continuously variable transmission is able to continuously change its speed ratio.
A belt-type continuously variable transmission is widely known as a continuously variable transmission for a vehicle. The belt-type continuously variable transmission includes a belt for transmitting power and a pair of pulleys each of which has a groove in which the belt is wound. The winding radius of the belt on each of the pair of pulleys increases or decreases with a change in the width of a corresponding one of the grooves. By changing the winding radii of the belt through changing the groove width of each of the pulleys, the continuously variable transmission steplessly changes its speed ratio that is set between those pair of pulleys.
In order to set a speed ratio higher than the maximum speed ratio of the continuously variable transmission or set a speed ratio lower than the minimum speed ratio of the continuously variable transmission, Japanese Patent Application Publication No. 3-61762 (JP 3-61762 A) describes the configuration of a transmission that combines a belt-type continuously variable transmission mechanism with a gear-type stepped transmission mechanism. In the transmission having the configuration described in JP 3-61762 A, by changing a power transmission path between the continuously variable transmission mechanism and the stepped transmission mechanism, it is possible to set the speed ratio higher than the maximum speed ratio or the speed ratio lower than the minimum speed ratio. The maximum speed ratio and the minimum speed ratio are allowed to be set in the continuously variable transmission. As a result, it is possible to expand the width of speed ratio that is allowed to be set by the transmission as a whole.
A controller for the transmission described in JP 3-61762 A employs the above-described transmission as a controlled object. In the transmission, a continuously variable transmission mechanism and a stepped transmission mechanism are provided in parallel with each other. In changing from a stepped transmission path to a continuously variable transmission path, a shift in the continuously variable transmission mechanism is prohibited until the change completes. The stepped transmission path transmits power via the stepped transmission mechanism. The continuously variable transmission path transmits power via the continuously variable transmission mechanism.
As described above, by forming the transmission in which the stepped transmission mechanism is provided in parallel with the continuously variable transmission mechanism, it is possible to expand the width of speed ratio that is allowed to be set by the transmission as a whole. Conversely, when a shift is carried out in the continuously variable transmission mechanism at the time of changing the power transmission path between the continuously variable transmission mechanism and the stepped transmission mechanism, a deviation increases between the speed ratio that is set in the continuously variable transmission mechanism and the speed ratio that is set in the stepped transmission mechanism. As a result, there is a possibility that a shock in the change increases, driving force becomes insufficient after the change or the engine rotation speed increases more than necessary.
Facing such problems, the controller for the transmission described in JP 3-61762 A keeps the speed ratio by prohibiting a shift in the continuously variable transmission mechanism in the period until the change from the stepped transmission mechanism to the continuously variable transmission mechanism completes as described above, thus making it possible to reduce a shift shock during the change and obtain desired power performance.
The above-described change of the power transmission path between the continuously variable transmission mechanism and the stepped transmission mechanism is usually carried out with engagement or releasing operation of a clutch. For example, in the configuration of the transmission described in JP 3-61762 A, the power transmission path is changed by engaging a “high clutch 60”. At this time, when a shift is prohibited in the continuously variable transmission mechanism and the speed ratio is kept as described above, differential rotation increases between friction materials in the “high clutch 60 and the durability of the clutch may decrease. That is, the continuously variable transmission mechanism generally hydraulically controls an actuator that is operated in order to set a speed ratio or a transmitted torque capacity. Therefore, there is an inevitable response delay at the time of hydraulic control, and there is a limit in trackability of an actual speed ratio to a target value of the speed ratio. For example, when the vehicle rapidly decelerates and then re-accelerates or starts moving again, the speed ratio of the continuously variable transmission mechanism may not have returned to the maximum speed ratio. When a shift is prohibited in the continuously variable transmission mechanism and the speed ratio is kept in such a state, a deviation increases between the speed ratio of the stepped transmission mechanism and the speed ratio that is set in the continuously variable transmission mechanism. Thus, differential rotation also increases in the clutch that is engaged for changing the power transmission path. As a result, friction increases at the time when the friction materials of the clutch engage with each other, so the durability of the clutch decreases. In addition, an engagement shock also increases at the time when the clutch is engaged.